It is well known that carbohydrates and sugar crops are readily available, inexpensive, and renewable feed stocks for the chemical and food industries. It is also well known that dates are rich in sugars ranging from 65% to 80% on a dry weight basis mostly of inverted form (fructose and glucose in about equal amounts). Further, it has been recognized that only about 10% of the date production is utilized for human consumption while the rest is used as an ingredient in fodder or disposed off.
In many cases, pure sugars are required. For example, the food industry uses large quantities of high fructose corn syrup (HFCS) while pure glucose is used for medical purposes and in the manufacture of pharmaceuticals.
Problems in the production of fructose on a commercial scale have been encountered. For example, since fructose and glucose so closely resemble one another in physical and chemical characteristics most reagents and solvents fail to separate them satisfactorily. Accordingly, early processes fail to produce high purity fructose economically.
There is considerable current interest in the utilization of carbohydrates and sugar crop materials such as dates as readily available, relatively inexpensive, and renewable feed stocks for the chemical and related industries. Examples of the later trend are sugar derived biosurfactants and esters of sugars with fatty acids. The transformation of underivatized carbohydrates is still challenging due to their low solubility in almost any solvent other than water. The few exceptions, such as dimethylsulfoxide and dimethylformamide, have many undesirable characteristics and are not compatible with many intended applications of carbohydrate-derived products.
Chromatography is the method most commonly used for commercial sugar separations. It is currently applied to enrich fructose content in HFCS to separate maltotriose and other compounds from starch hydrolysate, and to separate sucrose and other compounds from molasses. Such a separation is a batch process with relatively low productivity and relatively low yields of the desired product and normally requires expensive installations. Attempts to simulate continuous operation such as the use of a complex valve system or a special column arrangement on a rotating disk that contains the connections are hampered by the complexity of the process and the associated equipment and the high operating cost. Alternative processes have been proposed to accomplish sugar separation, such as zeolite adsorption and reverse osmosis. Processes based on the chemical affinity of sugars include electrodialysis using borates to complex the sugars, ion exchange membranes and liquid membranes. Limited success obtained with these processes reveals the need to develop more efficient processes.
One approach to overcome such problems is disclosed in a U.S. Pat. No. 3,533,839 of Harra et al. As disclosed, a process for separating fructose admixed with glucose comprises treating the mixture thereof with anhydrates absolute ethanol containing anhydrous calcium chloride to extract the fructose as an anhydrous addition compound with calcium chloride and to leave the glucose unextracted. Water is added to the extract to precipitate the fructose as a hydrated addition compound of the calcium chloride and filtering this participate.
Fractionation of sugars is another approach to the separation of fructose and glucose. However, the fractionation of sugar is a relatively difficult and costly task especially when the constituents have very similar characteristics such as glucose and fructose.
It is also known that batch process are commonly used for separation of glucose and fructose contained in a feed solution by inputting such feed solution through a fixed bed of a cation exchange column and then followed by a de-ionized water addition. The separation is carried out through a so-called chromatography, which incorporates a long column packed with a stationary resin. The separation is achieved through a mass transfer phenomenon or mechanism wherein the eluent water is flowing through a part of the stationary resin together with the feed solution in a so-called mass transfer zone. The fructose contained in the feed solution is retained by the resin to a greater degree than glucose.
Chromatographic separation has also been used for the recovery of xylose from hydrolysates of natural materials such as birch wood, corn cobs and cotton seed hulls. The resin employed in the chromatographic separation is a strongly acid cation exchanger, i.e., sulfonated polystyrene cross-linked with divinylbenzene. The use of a strongly acid cation exchanger for separation of monosaccharides e.g. xylose from magnesium sulfite cook liquor is also known. The chromatographic separation has been carried out using a simulated moving bed. However, the separation of certain monosaccharides by using strong acid cation exchange resins is difficult.
Anion exchange resins have also been used for separating fructose from glucose. For example, an anion exchanger in a bisulfite form is used for the separation of sugars. Water is used as an eluent. However, the use of anion exchange resins does not result in a good xylose separation because xylose is overlapped by other sugars. The separation of fructose and glucose by an anion exchanger in a bisulfite or sulfite form is known. For example, poly (4-vinylbenzeneboronic acid) resins in the fractionation and interconversion of carbohydrates has been used. In this process water is used as an eluent. The best yield of fructose was achieved when the pH was high. The resins have been used to displace the pseudo equilibrium established in aqueous alkali between D-glucose, D-fructose and D-mannose to yield D-fructose. Surprisingly, it has been found that when using weakly acid cation exchange resins, an improved chromatographic separation of carbohydrates is obtained. In addition to other features, the order of separation seems to be affected by the hydrophobic/hydrophilic interactions of carbohydrates with the resin and an improved separation of carbohydrates is obtained. Other commonly known features in chromatographic separation of carbohydrates on ion exchange resins include e.g., ion exclusion and size exclusion. If the resin is in the hydrophilic form, the most hydrophobic monosaccharides seem to elute first and the most hydrophilic last. This results in a different elution order than previously found.
U.S. Pat. No. 6,258,176 of Ma teaches a process for producing HFCS, in which a mixed feed solution of glucose and fructose is obtained from the isomerization tower. It is employed as a subsequent unit operation for separating opponents of sugar mixture.
More boldly, this process is used for the continuous separation of the glucose and fructose solution mixtures to retrieve various grades of glucose and fructose solution mixtures and to elevate the concentration level of the separated fraction. Yet, when this process compares with traditional chromatographic processes, it has its ultimate object to consume less resin inventory and eluent water to gain the ultimate purity and higher concentration of glucose and fructose components with ultimate yield and lower production cost.
Limited successes obtained with the aforementioned processes reveal a need to develop more efficient processes.
Fortunately, a new class of compounds, ionic liquids has emerged in the last ten years that may become a key ally in meeting the twin challenges of efficient and environmentally benign chemical processing. They have the potential to revolutionize the way we think of and use solvents. The reason is, they act like good organic solvents, dissolving both polar and nonpolar species. In many cases, they have been found to perform better than commonly used solvents. In addition, ionic liquids are non-volatile and non-flammable. The wide and readily accessible range of ionic liquids with corresponding variation in physical properties offers the opportunity to design an ionic liquid solvent system optimized for a particular process.
A key feature of ionic liquids is that their physical and chemical properties can be tailored by judicious selection of cation, anion, and substituents. For example, a choice of anions such as halide (Cl−, Br−, I−) nitrate (NO3−), acetate (CH3CO2−) trifluoroacetate (CF3CO2−) triflate (CF3SO3−) and bis(trifluoromethylsulfonyl) imide (CF3SO2)2N−) can cause dramatic changes in the properties of ionic liquids. The water solubility of the ionic liquid can be controlled by the nature of the alkyl substituent on the cation. Increasing the length of the alkyl chain tends to decrease water solubility by increasing the hydrophobicity of the cation.
It is now believed that the methods for separating fructose and glucose in accordance with the present invention offer a more effective and efficient process for separating the two sugars and for recovering fructose and/or glucose from dates. Further, the methods for separating fructose and glucose from mixtures of the two sugars in accordance with the present invention have been shown to produce fructose or glucose with a purity of more than 99%. In addition, the process in accordance with the present invention has been found to have higher yields and lower production costs, and employs non-flammable and non-volatile and environmentally benign materials.